Self-oscillating sawtooth current deflection generator

ABSTRACT

Disclosed is an SCR-analogue, dual coupled transistor vertical oscillator for synching the vertical sweep in a video display with vertical synchronization input signals. Capable of stable, free-running oscillation at two, variable DC levels, the coupled transistor configuration in combination with a capacitor generates a precisely defined sawtooth voltage waveform for controlling vertical sweep and flyback. This combination provides an inexpensive, flexible means for synchronizing sweep drive circuitry operating at various DC coupled levels with a wide range of synchronization input signals.

CROSS REFERENCE TO RELATED PATENT APPLICATION

This application is related to, but in no way dependent upon, thefollowing application which is assigned to the assignee of the presentapplication: Ser. No. 077,432, filed Sept. 20, 1979 entitled "VERTICALDRIVE CIRCUIT FOR VIDEO DISPLAY" in the name of Gregory J. Beaumont andLloyd Matthews.

BACKGROUND OF THE INVENTION

This invention relates in general to vertical sweep signal processingfor a CRT in a video display and in particular to circuitry forgenerating a sawtooth current waveform in the deflection coils of a CRTfor controlling scan and retrace of an electron beam in a video display.

Since the present invention finds particular utility in the verticaldeflection circuit of a CRT in a video display, it will be particularlydescribed with reference to its use in such apparatus, although it willbe appreciated that its uses are by no means limited to suchapplications.

Deflection circuits utilized in television receivers, and in CRT videodisplays in general, synchronize the deflection signals used to controlthe sweep of the electron beam with synchronizing pulses recovered fromthe composite video signals received by the television receiver or videodisplay. These circuits typically make use of the charge-discharge cycleof a capacitor in generating a sawtooth current waveform having apredetermined period and magnitude. The ramp of the sawtooth currentwaveform is generally developed from the discharge of a capacitor whilethe capacitor is recharged during the retrace period. It is by means ofthis sawtooth current waveform imposed upon the CRT's deflection coilsthat the electron beam scans and retraces over the faceplate of the CRTat the appropriate times.

Various approaches to deflection circuit design and, in particular,synchronization oscillator design to achieve synchronization of electronbeam sweep with input synchronization pulses are disclosed in the priorart. Early attempts in this area utilized switching diodes incombination with a voltage source to alternately charge and dischargethe capacitor. These efforts were followed by the incorporation oftransistors in CRT sawtooth current waveform generation circuits. Thesecomponents made the electron beam deflection circuits more reliable,more economical and, in general, better performing in terms of switchingspeeds and power consumption. These transistorized sawtooth deflectiongenerators, however, suffered from characteristic operating limitationsimposed on the current and the voltage of transistor deflectioncircuits. These performance limitations gave rise to the development ofsilicon controlled rectifier (SCR) circuits formed of a semi-conductorassembly capable of operating at higher currents than that of normalrectifier which has been modified to "block" in the forward directionuntil a small signal is applied to the control electrode, or gate. Afterthe control signal is applied, the device conducts in the forwarddirection with a forward characteristic very similar to that of a normalsilicon rectifier and continues to conduct even after the control signalhas been removed. In this manner, a multivibrator combination may bemade for providing a stable oscillator for synchronizing sweep signalswith synchronization input signals. The SCR-type oscillator-deflectioncircuits were not without limitations, however. Among their performancelimitations were instabilities, or drift, in the signal voltage levelsrequired to initiate the transistion to a stable oscillating state.Another inherent operating limitation in SCR oscillators is therequirement for an outside source of high power signals to terminate theSCR's oscillatory state.

One approach to the design of an SCR sawtooth wave generator isdisclosed in U.S. Pat. No. 4,122,363. Disclosed therein is a circuitarrangement for obtaining a periodic sawtooth current in a coilutilizing a controllable switching device, or SCR, whose controlelectrode is connected to a source of periodic drive pulses which renderthe switching device conductive during part of the sawtooth period.While taking advantage of the reliability and ease of firing ofthyristors, this invention requires a synchronization drive input pulseof a uniform width during each deflection cycle in order to initiateretrace. A lack of uniformity in synchronization input drive pulse widthfrom one cycle to another can result in improper operation of thedeflection circuit.

A transistorized vertical sawtooth deflection generator is disclosed inU.S. Pat. No. 3,435,282. This system provides a transistorized,self-oscillating vertical deflection generator utilizing a singlecapacitor of small value. Two coupled transistors form aself-oscillating network for charging a capacitor in a manner such thatcharge is being replaced on the negative terminal of the capacitorduring discharge which has the effect of prolonging its discharge timeand linearizing its rate of discharge. By also connecting the capacitorto a source of rising voltage during the trace interval the discharge ofthe capacitor is thereby impeded thus making it possible to use acapacitor of relatively small value while still achieving a slow rate ofdischarge. This system, however, was not designed to synchronize aDC-coupled vertical drive circuit with synchronization drive inputsignals of varying voltage amplitude and pulse width.

Still another approach to the design of a sawtooth current waveformgenerator for a CRT deflection system is disclosed in U.S. Pat. No.4,110,666. This circuit makes use of two SCR's in combination one ofwhich is turned on by the input synchronization pulses with the otherturned on by pulses applied just before the end of line scan. The firstSCR provides for electron beam scan synchronization while the second SCRprovides for proper retrace of the electron beam. This arrangementsuffers from the aforementioned trigger pulse turn-on instabilities anddrift while being limited in the synchronization input signal pulsewidth with which it can operate.

The present invention, however, overcomes these limitations by providinga self-oscillating transistorized circuit possessing a predictabletriggering level and capable of generating a sawtooth waveform of agiven period and current, or voltage, level from input synchronizationsignals of variable pulse width and voltage level.

OBJECTS OF THE INVENTION

Accordingly, it is an object of the present invention to provide animproved system for synchronizing the vertical sweep of an electron beamin the CRT of a video display with synchronization drive input signalsprovided to the video display.

Another object of the present invention is to provide an improvedvertical oscillator for driving DC-coupled, vertical drive circuitrywith a great variety of vertical drive synchronization input signals.

Still another object of the invention is to provide an improvedfree-running oscillator for generating a precisely controlled sawtoothcurrent waveform for driving vertical sweep circuitry of a CRT in avideo display.

BRIEF DESCRIPTION OF THE DRAWINGS

The appended claims set forth those novel features believedcharacteristic of the invention. However, the invention itself, as wellas further objects and advantages thereof, will best be understood byreference to the following detailed description of a preferredembodiment taken in conjunction with the accompanying drawings in which:

FIG. 1 is a circuit diagram illustrating the basic construction of aself-oscillating sawtooth current deflection generator for a CRTdeflection system embodying the principles of the present invention; and

FIGS. 2A thru 2D show a set of waveforms indicating the current andvoltage at several points in the self-oscillating sawtooth currentdeflection generator.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the circuit diagram of FIG. 1, there is shown aself-oscillating sawtooth deflection generator 10 in accordance with thepreferred embodiment of the present invention. Vertical drive negativesynchronization input pulses enter the system via pin no. 1 of the edgeconnector. The sawtooth waveform generator 10 is powered by a +15 voltDC supply. As shown in the figure, in the preferred embodiment of thepresent invention the output of the sawtooth current generator 10 isprovided to later stages of the vertical drive circuit (not shown), suchas the vertical amplifier and the vertical driver (also not shown). Thelater stages of the vertical drive circuit with which the presentinvention interfaces are described in patent application Ser. No.077,432, filed Sept. 20, 1979, entitled "VERTICAL DRIVE CIRCUIT FORVIDEO DISPLAY" in the names of Gregory J. Beaumont and Lloyd Matthews,which is assigned to the assignee of the present application. It is theoutput of the vertical drive circuit which controls the verticaldeflection yoke.

A pulse waveform of TTL-level logic input on pin no. 1 is used totrigger the sawtooth current deflection generator 10. A differentiationnetwork comprised of capacitor 12 and resistor 14 isolates the leadingedge of the input pulses by emitting a pulse corresponding to theleading edge of an input pulse which is used to trigger the sawtoothcurrent generator 10. This differentiation network generates a positivepulse for a positive-going input pulse edge and a negative pulse when anegative-going edge of an input pulse is received. The sawtooth currentgenerator, however, is triggered only be a negative-going pulse becausethe input is provided to the base of transistor 16 which results in theturn-on of this transistor. The input pulses can vary in width andamplitude because of the fact that the sawtooth generator 10 isresponsive to the leading edge of an input pulse. Thus, in the preferredembodiment of the present invention the vertical drive synchronizationinput pulses may vary from 2.5 to 5.5 volts and 50 microseconds to 1.4milliseconds in pulse width. These performance parameters are merelyrepresentative numbers which may be varied by changing component valuesin the present invention.

Capacitor 18 filters out high frequency, extremely narrow pulses whichcould erroneously trigger the sawtooth current generator 10. Resistor 20performs a signal limiting function so that input signals of the properamplitude range are provided to the sawtooth current generator. Thefrequency range of input signals with which the preferred embodiment ofthe present invention may operate is 47 to 63 H_(z). Again, thesenumbers are merely representative values of performance parameters whichmay be modified by changing component values or by minor modificationsin circuit design.

In accordance with the present invention, resistors 22 and 24 incombination with the +15 volt DC supply establish a voltage level on theoscillator network 26 formed by transistor 16 and transistor 28 at whichthe oscillator will trigger. This oscillator network 26 will free rununtil it receives a synchronization drive input signal of sufficientvoltage level on the base of transistor 16. The synchronization driveinput pulse is not seen on the emitter of transistor 16 because of theRC-filtering network consisting of capacitor 30 and resistor 22 acrosswhich the B+ supply of +15 volts DC is applied. Thus transistor 16 actsas a PNP-type transistor with a DC power supply coupled to its emitterso that transistor turn-on is initiated when the base voltage dropsbelow the emitter voltage. Transistor 16 is turned on, or conducts, whena negative input pulse of at least 0.6 volts is applied to its base.When transistor 16 turns on emitter and collector current start to flowtherein which, in turn, feeds the base of transistor 28. Positivecurrent pulses generated at the collector of transistor 16 andtransmitted to the base of transistor 28 cause transistor 28, which isan NPN-type transistor, to begin conducting current. This signal inputto the base of transistor 28 results in current being pulled from thecollector of transistor 28 resulting in more base current fromtransistor 16 which turns this transistor on harder. With transistor 16turned on harder, transistor 28 is also turned on harder and theoscillator network, or multivibrator, 26 acts as a regenerative circuitin the sense that transistors 16 and 28 turn each other on and reinforceeach other once turned on. Similarly, when one transistor stopsconducting, the other transistor of the oscillator network also turnsoff. The transistorized oscillator network 26 thus acts as an SCRanalogue circuit in which a first input pulse initiates oscillation anda second input pulse terminates oscillation.

A voltage divider network comprised of resistors 22, 24 and 32establishes two voltage levels on the oscillator network 26. A highvoltage level is established at the junction of resistors 22 and 32while a low voltage level is established at the junction of resistors 22and 24. The oscillator network 26 charges up to the voltage levelestablished by resistors 22, 24 and 32 with the high voltage levelestablishing the upper voltage across the oscillator network and thelower voltage level establishing the lower voltage across the oscillatornetwork during operation.

With the lower voltage level of the oscillator network 26 established bythe voltage at the junction of resistors 22 and 24, when an input pulseis delivered to the base of transistor 28 initiating current flow anincreasing voltage level is established across the oscillator network26. Because of the self-regenerative characteristic of the oscillatornetwork's transistor configuration, current flow increases in theoscillator network 26 until the voltage across the oscillator networkreaches the high voltage level established at the junction of resistors22 and 32. During this period of increasing current flow and voltagebuildup capacitor 34 is charged by the emitter current of transistor 28.When the oscillator network 26 charges up to the high voltage levelestablished at the junction of resistors 22 and 32 current ceasesflowing through the circuit with charge no longer being delivered tocapacitor 34. The collector of transistor 28 thereupon stops conductingwith capacitor 34 fully charged causing the base current of transistor16 to cease. The oscillator network 26 has then turned off, ceasing toconduct and resets itself for the next negative input pulse on the baseof transistor 16.

After capacitor 34 charges up and transistors 16 and 28 stop conducting,the vertical size control network 36 begins to discharge capacitor 34negatively. The charging of capacitor 34 by means of the oscillatornetwork 26 followed by the negative discharge of capacitor 34 by meansof the vertical size control network 36 results in a sawtooth waveformbeing generated by the system. With capacitor 34 fully charged andtransistors 16 and 28 turned off, current cannot pass through theoscillator network 26. Thus, capacitor 34 discharges through thevertical size control network 36 which is comprised of dischargeresistors 38 and 40 and vertical size potentiometer 42. The verticalsize control network 36 thus serves not only as a discharge path forcapacitor 34, but in so doing, also, in conjunction with the synchpulse, establishes the lower voltage limit of the sawtooth outputwaveform. Discharging resistors 38, 40 and 42 are thus instrumental inshaping the discharging voltage waveform of capacitor 34.

As capacitor 34 charges up transistor 16 goes into saturation. Becausethere is only a very slight voltage drop across the 47 ohms of resistor44, the saturation voltage is applied across the emitter-collectorcombination of transistor 16. Voltage builds up across the base oftransistor 16 at the same rate as capacitor 34 is charged. The base oftransistor 16 follows the emitter voltage upward, but displaced 0.6volts below it, and when capacitor 34 discharges, it comes down suddenlyto the DC voltage level established by the voltage divider network ofresistors 22 and 24. The voltage drop across transistor 16 is 0.2 voltswhile the voltage drop across transistor 28 is 0.6 volts.

Initially capacitor 34 is at ground when the system is turned on.Capacitor 34 begins to charge up when transistor 16 and transistor 28are turned on and continues to charge until the base voltage and theemitter voltage of transistor 28 get to within 0.6 volts of each other.Thereupon, transistor 28 is turned off so that the voltage to whichcapacitor 34 is charged is equal to the voltage at the junction ofresistor 22 and resistor 32 minus the saturation voltage of transistor16, the base-emitter junction voltage of transistor 28 and the voltagedrop across the 47 ohms of resistor 44 (which is essentiallynegligable).

The operation and performance of the sawtooth current generator 10 canbe better understood by reference to FIGS. 2A thru 2D. FIG. 2A shows thevertical drive negative synchronization pulses input on pin no. 1. FIG.2B shows the pulses corresponding to the negative-going andpositive-going edges of the vertical drive negative synchronizationpulses present at point A in the circuit. These positive and negativepulses are generated by the differentiation network comprised ofcapacitor 12 and resistor 14. It is the negative pulses which are seenby the base of transistor 16. FIG. 2C shows the positive input pulses tothe base of transistor 28 from the collector of transistor 16. Finally,FIG. 2D shows the voltage on the emitter of transistor 28 and thecorresponding buildup of charge and subsequent discharge of capacitor34. The high voltage level of the curve shown in FIG. 2D is determinedby the voltage divider network comprised of resistors 22, 24 and 32,while the low voltage level is determined by the vertical size controlnetwork 36, which is comprised of resistors 38, 40 and 42, and the timeinterval between synch pulses.

By way of illustrative example and not by way of limitation, thefollowing components of the preferred embodiment of the presentinvention may have the following values:

    ______________________________________                                        Reference No.    Preferred Value                                              ______________________________________                                        12               0.01    microfarads                                          14               22      kilohms                                              18               0.0033  microfarads                                          20               6.8     kilohms                                              22               9.1     kilohms                                              24               22      kilohms                                              30               22      microfarads                                          32               39      kilohms                                              34               0.15    microfarads                                          38               820     kilohms                                              40               1.5     megohms                                              42               250     kilohms (variable)                                   44               47      ohms                                                 46               200     kilohms                                              ______________________________________                                    

There has thus been provided a self-oscillating sawtooth currentdeflection generator for synching the vertical sweep of a CRT in a videodisplay with vertical drive synchronization input signals. By couplingtwo transistors in a current-regenerative configuration, a stable,inexpensive, free-running oscillator is formed which is triggered byprecisely defined voltage levels for charging a capacitor, the output ofwhich is a sawtooth waveform.

In addition, while particular embodiments of the invention have beenshown and described, it will be obvious to those skilled in the art thatchanges and modifications may be made without departing from theinvention in its broader aspects and, therefore, the aim in the appendedclaims is to cover all such changes and modifications as fall within thetrue spirit and scope of the invention.

I claim:
 1. A self-oscillating deflection generator for producing asawtooth current waveform in a magnetic deflection coil of a cathode raytube comprising:pulse differentiation means coupled to a source of inputpulses for generating a series of output pulses, each of said outputpulses representing the leading edge of each of said input pulses; firsttransistor means coupled to said pulse differentiation means forreceiving said output pulses; a DC voltage source coupled to said firsttransistor means so as to maintain said first transistor means at afirst voltage level with said first transistor means rendered conductiveupon receipt of said output pulses; second transistor means coupled tosaid DC voltage source and to said first transistor means such that thecurrent output of said first transistor means is connected to the inputof said second transistor means with the current output of said secondtransistor means connected to the input of said first transistor meansso as to form a regenerative, oscillatory network; voltage divider meanscoupling said second transistor means with said DC voltage source so asto maintain said second transistor means at a second voltage level, saidsecond voltage level being less than said first voltage level; capacitormeans coupled to said second transistor means such that said capacitormeans is charged by the output of said second transistor means; anddischarge means coupled to said capacitor means for discharging saidcapacitor means when said capacitor means becomes fully charged and saidfirst and second transistor means stop conducting thereby generating asawtooth current waveform.
 2. A deflection generator according to claim1, wherein said voltage divider network establishes a selected secondvoltage level on said second transistor means thereby producing adesired peak-to-peak voltage output by said discharging capacitor means.3. A deflection generator according to claim 1, wherein said firsttransistor means is a PNP-type with the base of said first transistormeans being coupled to the output pulses and the collector of said firsttransistor means being coupled to the base of said second transistormeans, and said second transistor means is an NPN-type with thecollector of said second transistor means being coupled to the base ofsaid first transistor means and the emitter of said second transistormeans being coupled to said capacitor means.
 4. A deflection generatoraccording to claim 1, wherein said source of input pulses providesnegative pulses to said self-oscillating deflection generator with saidpulse differentiation means being responsive to said negative-goingleading edge of said input pulses in generating said output pulses.
 5. Aself-oscillating deflection generator for producing a sawtooth currentwaveform in a magnetic deflection coil of a cathode ray tubecomprising:pulse differentiation means coupled to a source of inputpulses for generating a series of output pulses, each of said outputpulses representing the leading edge of each of said input pulses; afirst PNP-type transistor means having a base coupled to said pulsedifferentiation means for receiving said output pulses; a DC voltagesource coupled to the emitter of said first transistor means so as tomaintain said first transistor means at a first voltage level with saidfirst transistor means rendered conductive upon receipt of said outputpulses; a second NPN-type transistor means having a base coupled to saidDC voltage source and to the collector of said first transistor meansand a collector coupled to the base of said first transistor means suchthat the current output of said first transistor means is connected tothe input of said second transistor means with the current output ofsaid second transistor means connected to the input of said firsttransistor means so as to form a regenerative, oscillatory network;voltage divider means coupling the base of said second transistor meanswith a DC voltage source so as to maintain said second transistor meansat a second voltage level, said second voltage level being less thansaid first voltage level; capacitor means coupled to the emitter of saidsecond transistor means such that said capacitor means is charged by theoutput of said second transistor means; and discharge means coupled tosaid capacitor means for discharging said capacitor means when saidcapacitor means becomes fully charged and said first and secondtransistor means stop conducting thereby generating a sawtooth currentwaveform.